A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
It is important that each patterned layer of the IC has a certain sufficient alignment or overlay with the previous patterned layer(s) on which the layer is located with an alignment error as low as possible. To obtain such certain alignment, the lithographic apparatus includes an alignment system which determines the positions of one or more marks on the semiconductor substrate. Typically, each mark is associated with an exposure field. Such an exposure field may include one or more target portions (or dies) depending on the actual set up of the lithographic process.
A mark may be embodied as a grating. In that case, the alignment system uses a light beam that is aimed at the grating. A diffraction pattern is generated by the interaction of the light beam and the grating of the mark. A sensor of the alignment system is arranged for measuring diffraction orders in the diffraction pattern to obtain information relating to the position of the mark relative to a reference position on the substrate. From the position of the mark, the position of the corresponding target portion is determined. Then, upon exposure of the previously patterned layer, the target portion can be positioned so as to have a minimum alignment error when a next patterned layer is imaged on the substrate.
When a wafer is aligned, alignment data are determined based on the information from the diffraction pattern and refer to the positions of the marks in a wafer grid. A grid model, such as a higher order grid model, is used and the exposure positions for transferring the pattern to respective target portions are calculated. Such a method is useful in the case in which only a few locations on the wafer are measured.
The trend at present, however, is to have many more alignments per wafer and obtain alignment data from as many exposure fields as possible. With the increase of detection speed this will eventually lead to the situation that each exposure field will be measured to obtain alignment position information related to that respective exposure field.
Accordingly, the precision of alignment is fully determined by the precision of the sensor. It is considered that the alignment reproducibility of the sensor is the best position information one can obtain. The precision of the alignment and overlay is thus limited to the alignment reproducibility of the sensor. However, this approach will not be sufficient for ongoing efforts to improve overlay of the patterned layer with the previously patterned layer, in particular in view of the trend of reduction of feature sizes in integrated circuits.